I disagree.. I think Jon is correct.
99.9% of the time your developing a site for a customer who has their own look and feel.... package masters are a right royal pain the in arse!
Perhaps a simple step would be to name all package masters '<package-name>-master.adp'
In that way developers who *really* wanted them can just do a
mv <package-name>-master.adp master.adp
the master.adp in the package can then contain a reference to the site-wide master and nothing more..
Sound workable?